The present invention relates to an apparatus using opposed "herringbone" type rotors for expanding a gaseous medium, more specifically to a ceramic helical rotor expander utilizing low expansivity ceramic and/or metal materials, and specifically to a double-ended helical-rotor expander constructed to operate at an inlet temperature above 1100.degree. C.
Positive displacement expanders (expansion engines) depend on precise and dependable enclosure of the working fluid, usually a hot gas, while it expands (and cools) within the elements of the machine in such a manner as to transmit a moving force to an external load. In the helical rotor expander, two fluted rotors mesh precisely together within intersecting cylinders (bores) so as to contain working gases in an expanding volume, exert direct pressure forces on the lobes of the rotors so as to cause forceful turning of the output shaft.
In the helical rotor expander, close clearances alone are used to contain the working gases. This puts a premium on precise and stable fit between the rotors and with the case. This is possible with common metals, steel, aluminum, etc. only within a material temperature range of a few hundred degrees.
The helical rotor expander has no reciprocating, sliding, or abrading components which cause unbalance or require lubrication, and all the supporting components (bearings and seals) are external to the working space. Thus, the helical rotor expander has a smooth operation and no drag or wear in the working space.
The problem with successful construction of helical rotor concept engines operating on combustion products has been distortion of the closely fitting components, rotors and case under the thermal expansion of metal components induced by exposure to high, but rapidly changing temperature of the burning and/or expanding gases. In the helical rotor machine, the flow of the hot gases through the expander is quasi-continuous such that exposed components are either heated to gas temperature if uncooled or extract substantial energy from the gases if cooled. The former would result in substantial distortions in the relatively high expansion of common materials and would require excessive clearances. The latter would result in a substantial compromise in performance and efficiency.
In the early helical type expanders, problems due to the thermal deformation of the rotors and casings, and losses due to leakage and cooling, rendered it practically impossible to utilize sufficiently high temperatures and to retain the small clearances required for efficient operation. Thus, the efficiency of these early helical rotor machines was low, and thus not capable of competing with other machines, such as the gas turbine.
However, with the continued improvement in low expansivity materials capable of withstanding higher temperatures, and means for their manufacture, the difficulties arising from the thermal deformations of previous concepts are being overcome. With the advent of ceramic materials and the capability of producing casings and rotors from these materials, the herringbone type rotor systems, be they compressors, pumps, or expanders, provide advantages and competition with other machines, such as the gas turbine. The development of the helical rotor systems is exemplified by U.S. Pat. No. 698,539 issued Apr. 29, 1902 to T. C. McBridge; U.S. Pat. No. 1,135,648 issued Apr. 13, 1915 to C. E. F. Ahlm et al.; U.S. Pat. No. 2,289,371 issued Jul. 14, 1942 to A. Lysholm et al.; U.S. Pat. No. 2,410,172 issued Oct. 29, 1946 to A. Lysholm; U.S. Pat. No. 2,799,253 issued Jun. 16, 1957 to T. I. Lindhagen et al.; U.S. Pat. No. 3,102,681 issued Sep. 3, 1963 to H. R. Nilsson; U.S. Pat. No. 3,307,453 issued Mar. 7, 1967 to H. R. Nilsson et al.; and U.S. Pat. No. 3,881,849 issued May 6, 1975 to R. Commarmot et al.
As pointed out above, with the development of ceramic helical rotor machines in the early 1960's, efforts have been going forward to develop machines using the helical rotors. Ceramic-materials have sufficient strength at high temperatures and low expansivity and heat conductivity as compared with aluminum or steel, for example, and which further can be molded and machined with great accuracy. Of the above-referenced prior art approaches U.S. Pat. Nos. 3,307,453, and 3,881,849, utilize ceramic materials, with U.S. Pat. No. 3,307,453 teaching the use of ceramic materials in both the rotors and chamber walls. As pointed out above, it is widely recognized that in helical rotor machines, the efficiency of operation is largely dependent upon the closeness of the clearances between the rotors and between the rotors and the housing, and the precision with which these relationships can be maintained in service. This in turn is dependent upon the materials and accuracy of construction of the rotors and housing, and upon the bearings for supporting the rotors in a centered and aligned arrangement with respect to each other and the casings in which the rotors operate and the maximum operating temperature.
Thus, there has been a long felt need for materials which can withstand the high temperatures and which can be machined with accuracy. The above-referenced need exists in helical rotor expanders capable of operation at higher (i.e. 1100.degree. C.) inlet temperatures, useful for many applications in both propulsion, auxiliary and stationary power sources, and also for larger scale applications when the manufacturing capability is realized.
The present invention provides a helical or herringbone rotor expander particularly useful in applications for economical engines due to the present capability to manufacture such expanders. More specifically, the present invention involves a ceramic helical expander comprising a double-ended unit with hot gas entering at mid-length on one side and splitting to exhaust at both ends on the opposite side, and eliminates the problem relative to location of bearings and seals near the hot gas inlet. The helical rotor expander of this invention is of low expansivity, but high temperature material construction, made primarily of reaction bonded silicon nitride, for example. Also, the expander of this invention has the capability of operating at an inlet temperatures above 1100.degree. C. Thermal distortions are made tolerable by resort to the low expansivity materials, principally refractory, e.g. ceramic, materials. This principle has been successfully demonstrated by a 1500.degree. C. inlet temperature helical rotor expander made of graphite, a low expansivity refractory material, but not compatible with combustion gases. With the development and application of low expansion and combustion product resistant ceramics, such as silicon nitride, and the means for ready manufacture of the casings and herringbone rotors from this material, applications involving combustion products are now possible.